1. Field of the Invention
The invention relates to an active fixation cardiac electrode lead. In particular, the invention relates to an active fixation cardiac electrode lead for use with a cardiac pacemaker or an implantable cardioverter/defibrillator or a combination thereof.
2. Description of the Related Art
Electrode leads are e.g. used in combination with implantable medical devices like cardiac pacemakers or implantable cardioverter/defibrillator and serve for conducting electrical energy to an intracardiac location of a heart to be stimulated or defibrillated. Electrode may be specifically designed to be placed in a chamber of a heart like the right atrium or the right ventricle.
Typically, such electrode lead comprises one or more electrodes close to a distal end of the electrode lead. each electrode is electrically connected to a connector at the proximal end of the electrode lead via an electrical conductor. The connector serves for connecting the electrode lead to the implantable medical device. In order to keep the distal end of the electrode lead in a fixed position within the heart, active fixation means may be provided at the electrode lead's distal end. A typical active fixation means is a screw-in fixation helix similar to a miniature corkscrew.
In a typical embodiment, the fixation helix is movably placed in a housing at the electrode lead's distal end. The fixation helix can be extended out of the housing's distal end and retracted into an inner space enclosed by the housing by rotating the fixation helix around a electrode lead's longitudinal axis. Longitudinal movement of the fixation helix is mediated by a protrusion, e.g. a pin, fixed to the wall of the housing protruding inwards so as to extend into interspaces between windings of the fixation helix. Thus, the protrusion causes an axial movement of the helix when the helix is rotated around its longitudinal axis.
One example of an electrode lead having a helical fixation element for active fixation of a distal end of an electrode lead to the myocardium is disclosed in U.S. Pat. No. 4,886,074. The fixation helix is rotatably mounted to the distal end of an electrode lead. A pin fixed to the electrode lead's sleeve protrudes into the interspaces of the helix's windings for causing an axial movement of the fixation helix when the fixation helix is rotated. Rotation of the helix is achieved by a drive shaft extending from the electrode leads proximal end to the helix at the electrode lead's distal end.
The pin that interacts with the fixation helix can for example be integrated into the molded housing. In a housing made of metal the pin is an additional mounted component which is fixed to the housing for example by way of welding; see figure “PRIOR ART”
There are several disadvantages of the prior art:
During the extension of the helix into the myocardium tissue can be drawn into the housing of the fixation and can get caught by the pin. In case of repositioning of the lead or explantation the tissue can be hurt by tearing this caught portion apart.
For a strong fixation of the helix in the myocardium the helix should have a large diameter which is in conflict with a desired low diameter of the lead.
Known pins need a large wall thickness of the housing. The diameter of the fixation helix is limited to the inner diameter of the housing. So the wall thickness limits the diameter of the fixation helix.
Housings made of plastic need a larger wall thickness than metal housings because of their material instability. Housings made of metal need a large wall thickness for mounting the pin.
A conventional design providing a pin as an additional design element, for example a pin welded to a housings made of metal is an additional risk for the patient if the pin gets lost (risk of embolism).